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shm_toc.c
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1 /*-------------------------------------------------------------------------
2  *
3  * shm_toc.c
4  * shared memory segment table of contents
5  *
6  * Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group
7  * Portions Copyright (c) 1994, Regents of the University of California
8  *
9  * src/backend/storage/ipc/shm_toc.c
10  *
11  *-------------------------------------------------------------------------
12  */
13 
14 #include "postgres.h"
15 
16 #include "port/atomics.h"
17 #include "storage/shm_toc.h"
18 #include "storage/spin.h"
19 
20 typedef struct shm_toc_entry
21 {
22  uint64 key; /* Arbitrary identifier */
23  Size offset; /* Offset, in bytes, from TOC start */
25 
26 struct shm_toc
27 {
28  uint64 toc_magic; /* Magic number identifying this TOC */
29  slock_t toc_mutex; /* Spinlock for mutual exclusion */
30  Size toc_total_bytes; /* Bytes managed by this TOC */
31  Size toc_allocated_bytes; /* Bytes allocated of those managed */
32  uint32 toc_nentry; /* Number of entries in TOC */
33  shm_toc_entry toc_entry[FLEXIBLE_ARRAY_MEMBER];
34 };
35 
36 /*
37  * Initialize a region of shared memory with a table of contents.
38  */
39 shm_toc *
40 shm_toc_create(uint64 magic, void *address, Size nbytes)
41 {
42  shm_toc *toc = (shm_toc *) address;
43 
44  Assert(nbytes > offsetof(shm_toc, toc_entry));
45  toc->toc_magic = magic;
46  SpinLockInit(&toc->toc_mutex);
47 
48  /*
49  * The alignment code in shm_toc_allocate() assumes that the starting
50  * value is buffer-aligned.
51  */
52  toc->toc_total_bytes = BUFFERALIGN_DOWN(nbytes);
53  toc->toc_allocated_bytes = 0;
54  toc->toc_nentry = 0;
55 
56  return toc;
57 }
58 
59 /*
60  * Attach to an existing table of contents. If the magic number found at
61  * the target address doesn't match our expectations, return NULL.
62  */
63 extern shm_toc *
64 shm_toc_attach(uint64 magic, void *address)
65 {
66  shm_toc *toc = (shm_toc *) address;
67 
68  if (toc->toc_magic != magic)
69  return NULL;
70 
72  Assert(toc->toc_total_bytes > offsetof(shm_toc, toc_entry));
73 
74  return toc;
75 }
76 
77 /*
78  * Allocate shared memory from a segment managed by a table of contents.
79  *
80  * This is not a full-blown allocator; there's no way to free memory. It's
81  * just a way of dividing a single physical shared memory segment into logical
82  * chunks that may be used for different purposes.
83  *
84  * We allocate backwards from the end of the segment, so that the TOC entries
85  * can grow forward from the start of the segment.
86  */
87 extern void *
89 {
90  volatile shm_toc *vtoc = toc;
91  Size total_bytes;
92  Size allocated_bytes;
93  Size nentry;
94  Size toc_bytes;
95 
96  /*
97  * Make sure request is well-aligned. XXX: MAXALIGN is not enough,
98  * because atomic ops might need a wider alignment. We don't have a
99  * proper definition for the minimum to make atomic ops safe, but
100  * BUFFERALIGN ought to be enough.
101  */
102  nbytes = BUFFERALIGN(nbytes);
103 
104  SpinLockAcquire(&toc->toc_mutex);
105 
106  total_bytes = vtoc->toc_total_bytes;
107  allocated_bytes = vtoc->toc_allocated_bytes;
108  nentry = vtoc->toc_nentry;
109  toc_bytes = offsetof(shm_toc, toc_entry) + nentry * sizeof(shm_toc_entry)
110  + allocated_bytes;
111 
112  /* Check for memory exhaustion and overflow. */
113  if (toc_bytes + nbytes > total_bytes || toc_bytes + nbytes < toc_bytes)
114  {
115  SpinLockRelease(&toc->toc_mutex);
116  ereport(ERROR,
117  (errcode(ERRCODE_OUT_OF_MEMORY),
118  errmsg("out of shared memory")));
119  }
120  vtoc->toc_allocated_bytes += nbytes;
121 
122  SpinLockRelease(&toc->toc_mutex);
123 
124  return ((char *) toc) + (total_bytes - allocated_bytes - nbytes);
125 }
126 
127 /*
128  * Return the number of bytes that can still be allocated.
129  */
130 extern Size
132 {
133  volatile shm_toc *vtoc = toc;
134  Size total_bytes;
135  Size allocated_bytes;
136  Size nentry;
137  Size toc_bytes;
138 
139  SpinLockAcquire(&toc->toc_mutex);
140  total_bytes = vtoc->toc_total_bytes;
141  allocated_bytes = vtoc->toc_allocated_bytes;
142  nentry = vtoc->toc_nentry;
143  SpinLockRelease(&toc->toc_mutex);
144 
145  toc_bytes = offsetof(shm_toc, toc_entry) + nentry * sizeof(shm_toc_entry);
146  Assert(allocated_bytes + BUFFERALIGN(toc_bytes) <= total_bytes);
147  return total_bytes - (allocated_bytes + BUFFERALIGN(toc_bytes));
148 }
149 
150 /*
151  * Insert a TOC entry.
152  *
153  * The idea here is that the process setting up the shared memory segment will
154  * register the addresses of data structures within the segment using this
155  * function. Each data structure will be identified using a 64-bit key, which
156  * is assumed to be a well-known or discoverable integer. Other processes
157  * accessing the shared memory segment can pass the same key to
158  * shm_toc_lookup() to discover the addresses of those data structures.
159  *
160  * Since the shared memory segment may be mapped at different addresses within
161  * different backends, we store relative rather than absolute pointers.
162  *
163  * This won't scale well to a large number of keys. Hopefully, that isn't
164  * necessary; if it proves to be, we might need to provide a more sophisticated
165  * data structure here. But the real idea here is just to give someone mapping
166  * a dynamic shared memory the ability to find the bare minimum number of
167  * pointers that they need to bootstrap. If you're storing a lot of stuff in
168  * the TOC, you're doing it wrong.
169  */
170 void
171 shm_toc_insert(shm_toc *toc, uint64 key, void *address)
172 {
173  volatile shm_toc *vtoc = toc;
174  Size total_bytes;
175  Size allocated_bytes;
176  Size nentry;
177  Size toc_bytes;
178  Size offset;
179 
180  /* Relativize pointer. */
181  Assert(address > (void *) toc);
182  offset = ((char *) address) - (char *) toc;
183 
184  SpinLockAcquire(&toc->toc_mutex);
185 
186  total_bytes = vtoc->toc_total_bytes;
187  allocated_bytes = vtoc->toc_allocated_bytes;
188  nentry = vtoc->toc_nentry;
189  toc_bytes = offsetof(shm_toc, toc_entry) + nentry * sizeof(shm_toc_entry)
190  + allocated_bytes;
191 
192  /* Check for memory exhaustion and overflow. */
193  if (toc_bytes + sizeof(shm_toc_entry) > total_bytes ||
194  toc_bytes + sizeof(shm_toc_entry) < toc_bytes ||
195  nentry >= PG_UINT32_MAX)
196  {
197  SpinLockRelease(&toc->toc_mutex);
198  ereport(ERROR,
199  (errcode(ERRCODE_OUT_OF_MEMORY),
200  errmsg("out of shared memory")));
201  }
202 
203  Assert(offset < total_bytes);
204  vtoc->toc_entry[nentry].key = key;
205  vtoc->toc_entry[nentry].offset = offset;
206 
207  /*
208  * By placing a write barrier after filling in the entry and before
209  * updating the number of entries, we make it safe to read the TOC
210  * unlocked.
211  */
213 
214  vtoc->toc_nentry++;
215 
216  SpinLockRelease(&toc->toc_mutex);
217 }
218 
219 /*
220  * Look up a TOC entry.
221  *
222  * If the key is not found, returns NULL if noError is true, otherwise
223  * throws elog(ERROR).
224  *
225  * Unlike the other functions in this file, this operation acquires no lock;
226  * it uses only barriers. It probably wouldn't hurt concurrency very much even
227  * if it did get a lock, but since it's reasonably likely that a group of
228  * worker processes could each read a series of entries from the same TOC
229  * right around the same time, there seems to be some value in avoiding it.
230  */
231 void *
232 shm_toc_lookup(shm_toc *toc, uint64 key, bool noError)
233 {
234  uint32 nentry;
235  uint32 i;
236 
237  /*
238  * Read the number of entries before we examine any entry. We assume that
239  * reading a uint32 is atomic.
240  */
241  nentry = toc->toc_nentry;
242  pg_read_barrier();
243 
244  /* Now search for a matching entry. */
245  for (i = 0; i < nentry; ++i)
246  {
247  if (toc->toc_entry[i].key == key)
248  return ((char *) toc) + toc->toc_entry[i].offset;
249  }
250 
251  /* No matching entry was found. */
252  if (!noError)
253  elog(ERROR, "could not find key " UINT64_FORMAT " in shm TOC at %p",
254  key, toc);
255  return NULL;
256 }
257 
258 /*
259  * Estimate how much shared memory will be required to store a TOC and its
260  * dependent data structures.
261  */
262 Size
264 {
265  Size sz;
266 
267  sz = offsetof(shm_toc, toc_entry);
268  sz += add_size(sz, mul_size(e->number_of_keys, sizeof(shm_toc_entry)));
269  sz += add_size(sz, e->space_for_chunks);
270 
271  return BUFFERALIGN(sz);
272 }
int slock_t
Definition: s_lock.h:912
Size number_of_keys
Definition: shm_toc.h:46
uint64 toc_magic
Definition: shm_toc.c:28
shm_toc * shm_toc_create(uint64 magic, void *address, Size nbytes)
Definition: shm_toc.c:40
#define SpinLockInit(lock)
Definition: spin.h:60
shm_toc_entry toc_entry[FLEXIBLE_ARRAY_MEMBER]
Definition: shm_toc.c:33
int errcode(int sqlerrcode)
Definition: elog.c:575
Size offset
Definition: shm_toc.c:23
Size shm_toc_estimate(shm_toc_estimator *e)
Definition: shm_toc.c:263
#define PG_UINT32_MAX
Definition: c.h:380
Definition: shm_toc.c:20
#define SpinLockAcquire(lock)
Definition: spin.h:62
#define ERROR
Definition: elog.h:43
uint32 toc_nentry
Definition: shm_toc.c:32
unsigned int uint32
Definition: c.h:296
#define ereport(elevel, rest)
Definition: elog.h:122
Size space_for_chunks
Definition: shm_toc.h:45
Size toc_total_bytes
Definition: shm_toc.c:30
slock_t toc_mutex
Definition: shm_toc.c:29
#define SpinLockRelease(lock)
Definition: spin.h:64
Size mul_size(Size s1, Size s2)
Definition: shmem.c:492
shm_toc * shm_toc_attach(uint64 magic, void *address)
Definition: shm_toc.c:64
Size add_size(Size s1, Size s2)
Definition: shmem.c:475
struct shm_toc_entry shm_toc_entry
#define BUFFERALIGN_DOWN(LEN)
Definition: c.h:636
Size shm_toc_freespace(shm_toc *toc)
Definition: shm_toc.c:131
#define Assert(condition)
Definition: c.h:670
#define pg_read_barrier()
Definition: atomics.h:161
size_t Size
Definition: c.h:404
void * shm_toc_allocate(shm_toc *toc, Size nbytes)
Definition: shm_toc.c:88
Size toc_allocated_bytes
Definition: shm_toc.c:31
e
Definition: preproc-init.c:82
void shm_toc_insert(shm_toc *toc, uint64 key, void *address)
Definition: shm_toc.c:171
int errmsg(const char *fmt,...)
Definition: elog.c:797
int i
#define pg_write_barrier()
Definition: atomics.h:162
#define BUFFERALIGN(LEN)
Definition: c.h:625
uint64 key
Definition: shm_toc.c:22
#define elog
Definition: elog.h:219
#define UINT64_FORMAT
Definition: c.h:339
void * shm_toc_lookup(shm_toc *toc, uint64 key, bool noError)
Definition: shm_toc.c:232
#define offsetof(type, field)
Definition: c.h:593